The invention relates to a difference-in-speed-dependent hydraulic coupling comprising a rotating drive casing, a hydrostatic displacement machine arranged therein, an output shaft and a friction clutch for connecting the output shaft to the drive casing, the occurrence of a differential speed between the drive casing and the output shaft producing, in a pressure chamber, a pressure which acts on the friction clutch, the pressure chamber being connected to a lower-pressure chamber via at least one valve.
Such couplings are used in different arrangements in the drive train of motor vehicles, for the direct transmission of a torque or for blocking a connected differential gear for the drive of the wheels of one axle or for the distribution of the drive torque between two axles. In each case, the hydrostatic displacement machine comprises an inner rotor and an outer rotor, which, in the case of a difference in speed between the drive casing and an output shaft, execute a relative movement and thus produce a pressure.
Such a coupling is known from U.S. Pat. No. 5,536,215, FIGS. 1 and 7. In this coupling, a valve formed by a small bimetallic plate is provided between the pressure chamber and a lower-pressure chamber, that is to say the coupling chamber, it being the intention for said valve to compensate for temperature-dependent changes in viscosity of the working fluid. However, apart from the notorious inaccuracy of bending bimetallic plates, the action of said valve is adversely affected by centrifugal forces, fluid pressure and further hydraulic forces.
The object of the invention is thus to configure a coupling of the generic type, with as straightforward and reliable a design as possible, such that temperature-induced fluctuations in viscosity are compensated for with a high level of accuracy and reliability.
This is achieved, according to the invention, by providing a throttle valve which is formed by a valve body which is guided in a valve chamber, is supported on an element of temperature-dependent length and has a control edge which interacts with a control opening of the valve chamber, the control opening producing the connection between the pressure chamber and the lower-pressure chamber.
The element of temperature-dependent length makes the temperature compensation independent of the centrifugal force, and provides, if the length is sufficient, a high level of accuracy and, together with the action of the control edges, sufficient force in order to maintain even high pressure. This renders possible a precisely reproducible throttle setting, which is essential to functioning because this determines the characteristics of the entire coupling unit.
The throttle valve is preferably formed in a valve plate in which the valve chamber is a bore which is located in an axis-normal plane and leads past the center axis at a small distance therefrom (claim 2). The valve plate requires only a very small amount of space in the axial direction, but nevertheless may contain a very long axis-normal bore in which a long element of temperature-dependent length is accommodated and the valve body is guided to good effect. Since the bore leads past the center axis at a small distance therefrom, the friction-producing influence of centrifugal force in the direction transverse to the movement direction of the valve body is low. The bore arranged in this way also provides space for an adjusting screw, with the result that the position of the valve body can be precisely adjusted.
In an advantageous embodiment, the valve plate is firmly connected to the drive casing, and the pressure chamber is formed between the valve plate and an annular piston which interacts with the friction clutch (claim 3). In another advantageous embodiment, the valve plate is firmly connected to the annular piston or is a part of the same, and the pressure chamber is between the rotating drive casing and the annular piston, which accommodates the hydrostatic displacement machine, the valve plate being provided on that side of the annular piston which is directed away from the friction clutch (claim 4). In both embodiments, the element of temperature-dependent length is at such a distance from the friction clutch that the frictional heat of the latter does not distort the thermal expansion of the element. It thus reacts only to the temperature of the fluid of which the change in viscosity is to be compensated for.
In a preferred development of the invention (claim 5),
a) the control opening of the valve chamber forms an annular chamber which encloses said valve chamber and is connected to the pressure chamber,
b) the valve body is a tube of which the side which is directed away from the element of temperature-dependent length forms the control edge and the side which is directed toward the element of temperature-dependent length has openings to the valve chamber, and
c) the valve chamber is connected to a central bore of the valve plate.
The annular chamber around the control opening and the tubular formation ensure that the pressure acts on the valve body from all sides. This also allows the control edge to be machined cleanly. Through the openings on the side and the flow connection of the valve chamber to a central bore of the valve plate, the fluid flows past the element of temperature-dependent length, with the result that said element can rapidly follow the changes in temperature of the fluid.
In a refinement of the idea of the invention, the control edge is formed such that the pressure prevailing in the pressure chamber is independent of the temperature (claim 6). This achieves a particularly precise temperature compensation, the control edge formed on the valve body being particularly straightforward to machine.
The element of temperature-dependent length may advantageously either be a metal bellows which undulates in longitudinal section (claim 7) or consist of a suitable plastic (claim 8).
It also lies within the framework of the invention for the valve plate to be provided with a further bore which is located in an axis-normal plane, contains a spring-loaded piston and is connected to the pressure chamber (claim 9). This means that, as it were at the same time, with very low outlay and similar advantages, it is also possible to achieve damping of hydraulic pressure surges which could adversely affect the accuracy of the temperature compensation. In the same way, the valve plate could have a pressure release valve which is connected to the pressure chamber (claim 10).